I don't think you can resize on the board itself, you just have to make the image smaller before uploading it.

I think overall it looks pretty good. The pose looks natural and well drawn. Her face seems a little strange to me, maybe it's her hair? It doesn't seem that you did a lot of line thickness variation, and this really stands out in her hair. To give the impression of hair, you really need to have some more thick to thin lines, and not one consistent thickness. Her horns are skewed on her head. Her head is tilting one way, but the way the horns are positioned, they're tilting the opposite way.

As far as the coloring, you have a nice palette going. I think the green plushy is a little distracting, however. She's clearly the focus, but you chose a contrasting color for this plushy, and it's pulling attention away from her. One other thing, which is a pet peeve of mine, are her boots. Things that are gray or black are not literally grayscale. They still have some color to them, even it's incredibly subtle. They will take on a bit of the color of their surroundings and the light, particularly if they are as shiny as the one's she is wearing. I guess when you're coloring things in a very cartoony way, it's fine since you're stylizing things anyway, but it does seem like you're attempting a more natural type of lighting in this picture.

Anyway, nice work.

__________________"But... how does it feel? To just stop breathing?"
"Strange. You realize... just how much... effort... it has been... all along."100 sketches in 100 daysSketchbook

I've never known how to properly variate line thickness. I've seen people do it, but I just can't get it. Do I randomly change the thickness, or are there certain spots where the lines should be thicker (like rules to the variation or something)?
I see what you mean about the green plushie, and it didn't hit me until you said it. It DOES contrast a lot. I should have made it orange (they're called Poporings in Ragnarok Online).
I caught myself thinking, "Hair doesn't look that way" or, "This doesn't look realistic enough" at several points during the drawing, then I had to remind myself, "You're drawing MANGA, foo'!"
Thank you very much for the critique.

Generally speaking, line weight is centered around the lines that are "facing away" from the light source, and how close they are to the viewer (closer gets thicker lines).

I agree with this, and the tutorial seems fine, but oddly states that this isn't necessarily true, even though the person does thicken lines in areas of shadow. I'd ignore that part in the beginning. Maybe she meant it's not the only reason to vary thickness, which I'd agree with.

For hair, in general it's probably best to start thicker at the base and taper off at the ends.

I forgot to mention in my original post, but the perspective is also strange in this picture. You have the bed at an angle to the viewer, but it does not recede properly in perspective, giving it the illusion that it is actually getting wider the further away it is.

__________________"But... how does it feel? To just stop breathing?"
"Strange. You realize... just how much... effort... it has been... all along."100 sketches in 100 daysSketchbook